I was playing with my good friend today. I was playing Jarvan IV, but every time I used my ultimate on Vayne, she would be able to tumble out. I hope this is a bug because, she shouldn't be able to tumble through my wall. It got quite annoying and we lost because she was able to do it every time.
It's intentional.
In general, there is no z axis in this game. So there's no way for the engine to recognize the difference between a tristana "jumping" out of the arena or vayne tumbling out, or ezreal blinking out. In all cases, the game only cares whether the terrain the skill is aimed on to is passable. Since jarvan's ult has such a thin wall, even a vayne tumble can reach the normal terrain outside.
